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Stock-based Compensation (Narrative) (Details) - CAD ($) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Stock Issued During Period, Shares, Conversion of Units | 10,000 | 150,000
Employee Common Share Purchase Plan [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | 4,000,000 |
Allocated Share-based Compensation Expense |  | $ 1,000,000
Employee Common Share Purchase Plan [Member] | Scenario 1 [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Employee Stock Purchase Plan (ESSP), Employer Matching Percentage | 20.00% |
Employee Common Share Purchase Plan [Member] | Scenario 2 [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Employee Stock Purchase Plan (ESSP), Employer Matching Percentage | 10.00% |
Employee Common Share Purchase Plan [Member] | Minimum [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Employee Stock Purchase Plan (ESSP), Employee Contribution | $ 25 |
Employee Common Share Purchase Plan [Member] | Minimum [Member] | Scenario 2 [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Employee Stock Purchase Plan (ESSP), Employee Contribution | 3,000 |
Employee Common Share Purchase Plan [Member] | Maximum [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Employee Stock Purchase Plan (ESSP), Employee Contribution | 8,000 |
Employee Common Share Purchase Plan [Member] | Maximum [Member] | Scenario 1 [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Employee Stock Purchase Plan (ESSP), Employee Contribution | 3,000 |
Employee Common Share Purchase Plan [Member] | Maximum [Member] | Scenario 2 [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Employee Stock Purchase Plan (ESSP), Employee Contribution | $ 8,000 |
Common Shareholders Dividend Reinvestment and Share Purchase Plan [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Dividend Reinvestment Plan Percentage Discount From Average Market Price Of Common Shares | 5.00% |
Stock Option Plan [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | 11,700,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 10 years |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Contractual Term, Employee Retirement Or Termination For Other Than Just Cause | 24 months |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Contractual Term, Employee Termination For Just Cause Or Resignation Or Death | 6 months |
Fair Value Assumptions, Historical Volatility Period | 5 years |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount | $ 1,000,000 | 2,000,000
Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Cash Received | 1,000,000 | 3,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value | $ 1,000,000 | $ 2,000,000
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit | $ 21.99 | $ 21.58
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range Upper Range Limit | $ 46.19 | $ 46.19
Stock Option Plan [Member] | First Anniversary [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | 25.00% |
Deferred Share Unit Plans [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount | $ 2,000,000 | $ 7,000,000
Stock Issued During Period, Shares, Conversion of Units | 1 |
Number Of Trading Days | 10 days |
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense | $ 1,000,000 | 2,000,000
Deferred Tax Liabilities, Regulatory Assets and Liabilities | 0 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding | $ 37,000,000 | 35,000,000
Deferred Share Unit Plans [Member] | Scenario 1 [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Deferred Share Unit Plan Percentage Of Value Of Actual Annual Incentive Award | 25.00% |
Deferred Share Unit Plans [Member] | Scenario 2 [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Deferred Share Unit Plan Percentage Of Value Of Actual Annual Incentive Award | 50.00% |
Performance Share Unit Plan [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount | $ 14,000,000 | 14,000,000
Number Of Trading Days | 50 days |
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense | $ 4,000,000 | $ 4,000,000
Performance-based Share Unit Plan Performance Cycle Term | 3 years |

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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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